docs: typo in acceleration element

"accel3d" was specified twice, the second one is obviously "accel2d"
(also from the text down below.

docs/formatdomain.html.in

@@ -3112,7 +3112,7 @@ qemu-kvm -net nic,model=? /dev/null
   <devices>
     <video>
       <model type='vga' vram='8192' heads='1'>
-        <acceleration accel3d='yes' accel3d='yes'/>
+        <acceleration accel3d='yes' accel2d='yes'/>
       </model>
     </video>
   </devices>